Uranium dioxide (UO2) is the most commonly used nuclear fuel in commercial nuclear reactors, and has the advantages of high melting point, good high temperature stability, good chemical compatibility with cladding materials and coolant, strong anti-radiation capability, low thermal neutron capture section as anon-fission combined element oxygen and the like. However, there is still a significant problem with the UO2, that is, the heat conductivity is low. The heat conductivity is only 2.8 W/m·k at 1000° C., resulting in a large temperature gradient in the UO2 fuel pellets under working conditions, thereby producing high center temperature and severe thermal stress that will lead to large-scale cracks of the fuel pellets. The cracks may significantly increase the release of fission gases under a high burn-up condition, while the increase of operation temperature of the reactors may further accelerate the release of the fission gases and form bubbles, thereby leading to the swelling of the fuel pellets. Therefore, the service life of the UO2 fuel in the reactors is obviously limited.
Furthermore, high center temperature and large temperature gradient of the fuel pellets may lead to a great amount of waste heat, which will apparently increase the cladding temperature in a failure accident of the coolant. For example, for a uranium dioxide fuel pellet of a zirconium alloy cladding, since a reaction rate of the zirconium alloy cladding and water may be greatly increased at a temperature of 1200° C. or higher, a great amount of ZrO2 may be generated on the surface, thereby reducing the heat transferring capability of the cladding, finally leading to the crack of a zirconium alloy cladding layer and resulting in safety potential danger (R. O. Meyer. Nucl. Technol., 2006. 155: 293). Therefore, in order to improve the safety of the nuclear reactor and the burn-up of the nuclear fuel and reduce the nuclear power cost, on the premise of not influencing characteristics of UO2 neutrons, it is of crucial importance to improve the heat conductivity of the UO2 nuclear fuel and the retention capability of a fission product in a radiation environment.
Research shows that a barrier of a crystal boundary is a key factor limiting the heat conductivity of UO2; the existence of the crystal boundary may lead to the decrease of the heat conductivity to a great extent; and at different temperatures, the heat conductivity of the UO2 is on a rising trend along with the increase of the crystal grain size (Nature Communications, 2014, 5: 1-7). Therefore, the preparation of the UO2 with large crystal grain size becomes an important means to improve the heat conductivity of the UO2 nuclear fuel.